Technical Field of the Disclosure
This invention relates generally to switched reluctance motor drive systems, and more particularly to a system for rotor position estimation based on the measurement of inductance of the phases of the switched reluctance motor.
Description of the Related Art
A switched reluctance motor (“SRM”) is a rotating electric machine where both stator and rotor have salient poles. The switched reluctance motor is a viable candidate for various motor control applications due to its rugged and robust construction. The switched reluctance motor is driven by voltage strokes coupled with a given rotor position. The SRM is a brushless electrical machine with multiple poles on both rotor and stator. The stator has phase windings, unlike the rotor which is unexcited and has no windings or permanent magnets mounted thereon. Rather, the rotor of an SRM is formed of a magnetically permeable material, typically iron, which attracts the magnetic flux produced by the windings on the stator poles when current is flowing through them. The magnetic attraction causes the rotor to rotate when excitation to the stator phase windings is switched on and off in a sequential fashion in correspondence to rotor position. For an SRM, a pair of diametrically opposed stator poles produces torque in order to attract a pair of corresponding rotor poles into alignment with the stator poles. As a consequence, this torque produces movement in a rotor of the SRM.
The use of switched reluctance motor drives for industrial applications is of recent origin. SRM drives have been considered as a possible alternative to conventional drives in several variable speed drive applications. In conventional SRMs, a shaft angle transducer, such as an encoder or a resolver, generates a rotor position signal and a controller reads this rotor position signal. In an effort to improve reliability while reducing size and cost, various approaches have previously been proposed to eliminate the shaft position sensor by determining the reference commutation angle. These approaches implement indirect rotor position sensing by monitoring terminal voltages and currents of the motor. The performance of a switched reluctance machine depends, in part, on the accurate timing of phase energization with respect to rotor position. These methods are useful when at least one phase is energized and the rotor is spinning.
Another approach describes a system and method for achieving sensorless control of SRM drives using active phase voltage and current measurements. The sensorless system and method generally relies on a dynamic model of the SRM drive. Active phase currents are measured in real-time and, using these measurements, the dynamic equations representing the active phases are solved through numerical techniques to obtain rotor position information. The phase inductances are represented by a Fourier series with coefficients expressed as polynomial functions of phase currents to compensate for magnetic saturation. The controller basically runs the observer in parallel with the drive system. Since the magnetic characteristics of the motor are accurately represented, the state variables, as computed by the observer, are expected to match the actual state variables. Thus, rotor position, which is also a state variable, will be available indirectly. This system teaches the general method for estimating rotor position using phase inductance measured from the active phase. Here, they apply voltage to the active phase and measure the current response to measure position. This current magnitude is kept low to minimize any negative torque generated at the shaft of the motor.
Another approach describes a method of indirect motor position sensing that involves applying voltage sensing pulses to one unenergized phase. The result is a change in phase current which is proportional to the instantaneous value of the phase inductance. Proper commutation time is determined by comparing the change in phase current to a reference current, thereby synchronizing phase excitation to rotor position. Phase excitation can be advanced or retarded by decreasing or increasing the threshold, respectively. Due to the unavailability of inactive phases during higher speeds, this commutation method which makes use of the inactive phases of the SRM is limited to low speeds. Furthermore, although current and torque levels are relatively small in an inactive phase, they will contribute to a loss in SRM efficiency in this application.
Yet another approach discloses a rotor position estimator for an SRM based on instantaneous phase flux and phase current measurements. Phase current and flux sensing are performed for the phases in a predetermined sequence that depends on the particular quadrant of SRM operation. For each phase in the predetermined sequence of sensing, phase flux and phase current measurements are made during operation in a pair of predetermined sensing regions, each defined over a range of the rotor positions. The rotor position estimates are derived from the phase flux and phase current measurements for each respective phase during the respective sensing regions thereof. The rotor position estimates for each phase are normalized with respect to a common reference phase, and a rotor position estimate for the SRM is computed according to an equation which accounts for the fact that for any given rotor position determined, the rotor poles of the SRM may be approaching alignment or misalignment. Sampled phase voltage and phase current are integrated to obtain phase flux.
There remains a need for a method of quasi-sensorless adaptive control of a switched reluctance motor drive using a unique sequence of relation between phase inductances to enhance the accuracy of rotor position estimation. This method would very tightly monitor the speed with as many as 30 updates per revolution, which would thus provide a higher resolution than several sensorless approaches currently in use. Such a needed method would automatically accommodate for motor-to-motor or process variations, since it would not assume complete uniformity among all manufactured machines. Further, this approach would create a control algorithm that would not need to be calibrated for all motor specifications and power ratings. Moreover, this method would be able to naturally calibrate the control algorithm to the inductance profile of the machine that is being tested. Such a system would not require any adjustment in the control algorithm and would not require any prior knowledge of manufacturing specifications of the motor, which would further reduce the constructional detail burden of the machine manufacturer. This approach would use its own set of steps for automatically calibrating the inductance profile for any machine and would thus save time and resources involved in setting up and testing of the machine in an industry setting. Finally, the method would be reliable, robust, and completely scalable and would provide a clear technique that actively seeks to calibrate the model to each machine that is manufactured. The present embodiment overcomes shortcomings in the field by accomplishing these critical objectives.